Engine Systems and Components
Domain 14.3 tests the supporting systems that let an engine start, run, stay cool and stop safely: fuel, oil, ignition and starting, air, indication, FADEC and thrust reversers. Questions are usually about sequence, redundancy, and what happens when a component fails.
Fuel system
Typical flow: LP pump → filter → HP pump (gear or plunger, positive displacement) → metering unit or FADEC → flow divider → nozzles. Fuel is also the primary heat sink, so it commonly passes through a fuel-cooled oil cooler before metering. Metering is by mass flow; volumetric figures must be corrected using specific gravity, which varies with temperature.
Lubrication
Dry-sump systems comprise tank, pressure pump, filter, cooler, scavenge pumps and chip detector. Oil lubricates, cools, cleans, seals and protects against corrosion. Low oil pressure immediately after start that recovers as oil warms is normal; pressure that stays low is not.
Starting and ignition
Air-turbine or electric starter, ignition exciters and igniters. The sequence - motoring, ignition on, fuel on, light-up, acceleration to idle - is monitored against EGT and N2 limits. A hot start (EGT climbing toward the limit with sluggish N2) calls for fuel off and continued motoring to purge and cool.
FADEC and reversers
Dual-channel FADEC with active/standby changeover schedules fuel, VSVs, bleed valves and variable turbine geometry, enforces limits and selects thrust ratings; some installations retain a hydromechanical backup. Reverser deployment is interlocked: throttle at idle, aircraft on ground, reverser unlocked, then deploy.
Worked example
An engine burns 2 400 kg/h; fuel SG is 0.78. Volumetric flow = 2 400 ÷ 0.78 = 3 077 L/h = 51.3 L/min. Refuelled with SG 0.80 at the same volumetric setting, mass flow becomes 2 462 kg/h - about 2.6% rich, shifting EGT and possibly tripping a limit. This is why mass flow, not litres, is the controlled variable.
Traps
Only one FADEC channel controls at a time; know which is active. Fail-fixed holds the last commanded position; fail-safe drives to a safe position. On many engines the igniters run only for start and are switched off at idle. Do not confuse fuel flow indication with fuel pressure indication.